Rapid technological innovations and developments in information access and retrieval have greatly improved functionality of optical media in recent years. As a result, reading speed of optical disk drives, which are used as computer peripheral devices, has also made great progress. Nowadays, CD-ROMs with spindle motor rotation speed exceeding 10,000 RPM have become mainstream products on the market.
When the spindle motor operates at high speeds, the centrifugal wobbling force caused by unbalanced weight of the disk also increases. As a result, greater vibrations and noises will occur.
When the optical disk drives are put to practical use, excessive vibration and noise will make the optical read-write head unstable and make the optical disk drives unable to read data at their maximum rotation speed. The noise generated while optical disk drives are operating at high speeds also becomes an annoyance to many users.
Hence, to reduce vibration and ensure that optical disk drives are capable of reading data accurately and smoothly at their maximum rotation speed has become a critical issue faced by optical disk drive producers these days.
There are generally three conventional approaches to reduce the vibration caused by unbalanced weight of the disk when optical disk drives operate at high speeds: (1) increase the weight of the reading mechanism to reduce vibration; (2) utilize a dynamic absorber based on vibration theory to dampen vibration; (3) employ an auto-balancing system to decrease vibration.
While these three approaches can reduce vibration of optical disk drives to various degrees, the auto-balancing system achieves the best result. It utilizes an additional balance weight to decrease the triggering force of the vibration.
FIGS. 1 and 2 illustrate a conventional auto-balancing system, which is widely adopted for optical disk drives at present. The auto-balancing system includes an auto-balancing apparatus 11a located above (or below) a spindle motor 10a. The auto-balancing apparatus 11a includes a balance weight 12a (which is usually a plurality of steel balls) and a housing 13a (usually a circular element) for holding the balance weight 12a. Based on rotor dynamics, when the rotation speed of the spindle motor 10a is greater than the critical speed, the additional weight is moved to the location of the inverse phase of an unbalanced vector of the original optical disk 14a. As a result, the unbalanced weight of the optical disk 14a is offset, and vibration of the optical disk drive is reduced.
In practical applications, the balance weight of the auto-balancing apparatus is moved to the inverse phase of the unbalanced vector of the original optical disk only when the rotation speed exceeds the critical speed. For read-only optical disk drives, the spindle motors mostly operate at very high rotation speeds (generally greater than 6,000 RPM) and the balance weight can be moved to desired locations without problems.
However for re-writable optical disk drives, operation of the spindle motor for writing data often encounters certain conditions. One of these conditions is that the spindle motor rotation speed could drop to 200-500 RPM due to optical disk or laser power factors. At low rotation speeds, the balance weight becomes very unstable and tends to roll easily. It could lose the effect of offsetting the unbalanced weight. Moreover, the rolling generates noise or produces interference to the spindle motor, and makes the server system unable to control the motor to rotate at steady speeds. Hence conventional auto-balancing apparatus are not applicable to re-writable optical disk drives.